In a single-shaft combined cycle plant, when a condenser is switched between normal washing and reverse washing, the flow of sea water stops momentarily, with the result that the degree of vacuum of the condenser decreases suddenly. The decreased degree of vacuum of the condenser decreases the output of a steam turbine, thus causing a situation where the output of a generator decreases.
The steam turbine output is conventionally calculated with correction based on the degree of vacuum of the condenser, but if the degree of vacuum of the condenser changes suddenly, an instrument cannot follow the change in degree of vacuum, and the steam turbine output may be erroneously recognized to remain unchanged by calculation despite the fact that the steam turbine output has actually decreased.
In general, the generator output is calculated as the sum of the steam turbine output and the gas turbine output; as described above, so when the generator output decreases while the steam turbine output apparently remains unchanged as described above, the gas turbine output is erroneously recognized to have decreased. This prevents proper control of the gas turbine and poses the risk of malfunctions of various equipment in the power plant.
To eliminate this problem, therefore, Patent Literature 1, for example, discloses a gas turbine control system capable of more accurately calculating the steam turbine output and more precisely setting the gas turbine output by adding correction of the degree of opening of a condenser reverse washing valve to the calculation of the steam turbine output to compensate for a delay in response from an instrument measuring the degree of vacuum of a condenser by using the degree of opening of the condenser reverse washing valve.